Microbial Life in Hypersaline Environments

At salinities greatly exceeding 1.5 M, prokaryotes predominate: The moderately halophilic and haloversatile bacteria at salt concentrations between 1.5 and 3.0 M and the extremely halophilic Archaea at concentration around the point of halite precipitation.

Halophiles in Hypersaline Environments

An aerial view shows the pink water of Great Salt Lake brushing up against the Eco-sculpture "Spiral Jetty" on a salt-crust shore. Image credit: Bonnie Baxter.

Great Salt Lake, second in salinity only to the warmer Dead Sea, was once considered equally devoid of life. The North arm of Great Salt Lake is saturated with dissolved salts (>30%), yet life has found a way in all such lakes. We now know that hypersaline bodies of water that exceed the modest 3.5 % salt of earth's oceans are populated with rich communities of "halophiles," or salt-lovers. These microbes are in all three of the Domains of life, Archaea, Bacteria, and Eukarya (Baxter et al., 2005, Felix and Rushforth, 1979), however, eukaryotes are in small numbers. The halophilic microbes are colored with carotenoid compounds in their cell membrane, painting the waters with a pink-orange hue. Some species also have a purple membrane, regions where bacteriorhodopsin (BR) or other rhodopsin-like chromo-proteins reside (DasSarma and Arora, 2001).

Overcoming the Challenge of a High-Salt Environment

Adapted from Gilmour, 1990.
Bacteriorhodopsin (BR) creates a proton gradient and allows Na+ to be pumped out of the cell. The pump is created by pushing one proton across the membrane for every photon absorbed. This proton gradient leads to the formation of ATP. The Na+ gradient then allows K+ to be taken into the cell and balance osmotic pressure. Halorhodopsin (HR) uses light in order to pump Cl- into the cells and balance the K+ ions within.

In general, nonhalophilic organisms tolerate NaCl levels up to 0.2 mol/L. Halophiles, however, thrive at NaCl levels of 3.5 mol/L. To prevent the loss of cellular water to the environment, halophiles accumulate solutes within the cytoplasm (Galinski, 1993). Halophilic archaea, by use of a Na+ pump, push Na+ ions out of the cell, while concentrating K+ ions within the cell in order to balance osmotic pressure. This balance consists of an internal concentration of K+ at around 5M and an outside concentration of Na+ at around 4M. Halotolerant Bacteria and algae balance osmotic pressure by producing, or taking from the environment, organic molecules such as glycerol to act as compatible solutes (Litchfield, 1998).

Overcoming the Challenge of Intense UV Radiation

Great Salt Lake water inoculated on media plates yields colonies boasting colorful carotenoids. These compounds contribute to photoprotection, allowing the cells to live in intense UV. Image credit: Bonnie Baxter.

Microbes that inhabit hypersaline lakes experience intense ultraviolet (UV). In order to survive in this type of environment, halophiles have efficient DNA repair, but they also have mechanisms to prevent damage. For example, halophilic Archaea have a low number of UV "targets," thymines, in their genomes. The colorful carotenoids may also be a strategy for photoprotection as mutant colorless halophiles are UV sensitive (e.g., Dundas and Larsen, 1963). These are a class of very important antioxidants that may provide protection from UV damage. Exposure to UV light is necessary for the activation of Bacteriorhodopsin, a purple chromoprotein located within the cell membrane, which acts as a proton pump and drives ATP synthesis. Halobacterium species produce gas vesicles which enable them to float to the surface of the water column where light and oxygen are readily available (DasSarma and Arora, 2001)

Overcoming the Challenge of Desication

These crystals were found among pieces of debris from the hypersaline Mono Lake. Their appearance reflects the very high concentrations of salts that occur in the lake. Image taken by David Patterson and provided courtesy of microscope .

The saline concentration of North Arm GSL is near the saturation level for sodium chloride (Litchfield, 1998). Evaporation at the surface of GSL results with the molecules of salt being closer together. These molecules form small crystals of salt that float on the surface of the brine. The edges of these tiny crystals, which are surrounded by the hypersaline lake water, grow by the addition of salt molecules. As the crystal becomes heavier, it floats lower and lower in the brine, which results with the pyramid-shaped crystal. Crystals eventually reach the surface of the lakebed, where they continue to grow (Wardlaw et al., 1966).

As the crystals grow, small pockets of brine are trapped within the salt structure. As the rate of crystal growth increases, the quantity of fluid inclusions also increases. Quantities of inclusions are greatest in the center of the crystal (Roedder, 1984). As a crystal forms, sometimes halophiles become trapped within the fluid inclusion of the halite crystals. These enclosed halophiles may remain viable in the inclusions for many years (Norton and Grant, 1988; Norton et al., 1993; Denner et al., 1994; Grant et al., 1998; Vreeland et al., 2000). The population of viable halophiles is hypothesized to decrease as resources are depleted over time (Norton and Grant, 1988).

Various species of halophilic Archaea (halophiles) have been revived from fluid inclusions in ancient salt crystals (Norton et al., 1993; Denner et al., 1994; Grant et al., 1998; Vreeland et al., 2000). A new species, Halococcus salifodinae, was one novel isolate discovered in an Austrian salt mine (Denner et al., 1994). Many different species were isolated from salt crystals in two British salt mines. Based on lipid patterns, three out of nine taxonomic groups of halophiles were isolated from both of the salt mines (Norton et al., 1993).

Meridiani Planum, The red cross marks the Opportunity Rover landing site on this Hubble photo taken during the Mars opposition of 2001. Image Credit: NASA and the Hubble Heritage Team (STScI / AURA)

The quantity of ancient crystals that harbor viable halophiles, however, is low. This is probably due to depletion of resources. In one study of 250 million year-old salt crystals (Vreeland et al., 2000), only two of the 52 crystals studied contained Archaea that survived dormancy. The effects of depleted resources can also be observed in recently formed salt crystals. Norton and Grant (Norton and Grant, 1988) found that rod-shaped halobacteria become spherical in shape within two or three weeks of crystal formation. This plieomorphism is typical of rod-shaped halophilic Archaea in a starved state.

Halophiles on Mars?

Great Salt Lake has high sodium chloride concentrations as well as a significant amount of sulfate. The same could be said for the evaporates discovered on the Meridiani Planum plains of Mars, a hypothesized salt lake. Since we know that halophiles can remain dormant for long periods of time, what would we find if we searched in salt crystals from the red planet?

References

Halophile Collections

General Collection: Resources such as news articles, web sites, and reference pages provide a comprehensive array of information about halophiles.

Advanced Collection: Compiled for professionals and advanced learners, this collection includes resources such as journal articles, academic reviews, and surveys.

For Educators: This collection includes activities, assignments, and reading materials created specifically for educators.

Hypersaline Environments

Mono Lake: Mono Lake, located in California's Eastern Sierra, is both alkaline and hypersaline. In addition to its unusual array of alkaliphilic, halophilic, and anaerobic inhabitants, it has a remarkarble preservation success story.

Additional Resources

For additional resources about Halophiles, search the Microbial Life collection.